So far, in the case of retaining or multiplying a vector DNA in E. coli, there occurs a problem that the vector is not retained or multiplied stably in the host because the vector DNA, especially the exogenous DNA, recombines and the structure of DNA changes. Therefore, recA mutant in which homologous recombination is drastically suppressed is often used for the host to retain a vector DNA or to express a foreign DNA. But in recA mutant host, nonhomologous recombination is not suppressed because nonhomologous recombination does not require RecA protein.